I want to implement a service in OSX such that I can easily shrink the size of image files in a folder to half of their original size.
This is what I have come up with at the end.
For non-standard action (e.g. copy files to a folder which is unknown at the workflow design time), I need to add an action to assign input value to a variable and then refer to it in subsequent task.
A lot of useful actions are already built in e.g. Scale image
Saving it to the default automator folder will automatically register the service to Finder.